In recent years a novel microscopy, called near-field optical microscopy has extended the range of optical measurements beyond the diffraction limit and stimulated interests in many disciplines, especially material sciences and biological sciences. In the most widely adapted aperture approach, light is sent down an aluminum-coated fiber tip of which the foremost end is left uncoated to form a small aperture.
Unfortunately, only a tiny fraction of the light coupled into the fiber is emitted through the aperture because of the cut-off of propagation of the waveguide modes. The low light throughput and the finite skin depth of the metal are the limiting factors for resolution. This applies also to optical lithography since a large photon flux has to be ensured. Nowadays it is doubted that an artifact-free resolution of 50 nm will be surpassed by the aperture technique. Many applications in nanotechnology, such as nanolithography or the optical characterization of semiconductor nanostructures, require higher spatial resolutions.
Moreover, the aperture technique has other practical complications: 1) it is difficult to obtain a smooth aluminum coating on the nanometric scale which introduces non-reproductibility in probe fabrication as well as measurements; 2) the flat ends of the aperture probes are not suitable for simultaneous topographic imaging of high resolution; 3) the absorption of light in the metal coating causes significant heating and poses a problem for temperature sensitive applications.
Despite these limitations, various proposals for nanolithography using the near-field aperture approach have been put forth. In these experiments, patterning on the order of ≈100 nm has been demonstrated in conventional photoresists, photosensitive polymers, ferroelectric surfaces, and hydrogenerated amorphous silicon photoresists. Near-field optical lithography is unlikely to become speed-competitive with emerging parallel exposure technologies, such as X-ray lithography, because of the mechanical resonances associated with the scanning process; a problem inherent to all scanning probe techniques. On the other hand, it provides the capability and advantage of simultaneous modification, imaging and surface characterization.